Researchers learn about molecules by hitting them with lasers, watching them flex

Researchers show how to use an AFM to observe optical absorption by molecules …

With all the technology at our disposal, you would think that looking at chemistry at the nanoscale would be a piece of cake. After all, we can see the fluorescence from single atoms, right? We can see supernova in distant galaxies; how can it be so hard to figure out how molecules are sitting next to each other on a surface?

Very hard, it turns out. This is because molecules are complicated beasts, and figuring who they are and what they are doing with their neighbors can be even more complicated. Normally, we might choose to do this with light. But when we get down to the level of a few molecules, the signals are so weak that they are very hard to detect. Now, a very clever combination of atomic force microscopy and infrared spectroscopy has shown great promise for looking at molecules at very small scales.

When it comes to figuring out molecules through their interaction with light, we basically have two options: direct absorption spectroscopy and Raman spectroscopy. Atoms and molecules absorb certain colors of light, and this can tell us a lot about them. The high energy light (visible and ultraviolet) is absorbed by electrons moving to new energy levels, but this is not very useful for understanding molecules.

Instead, we look in the infrared, for light frequencies that will set the atoms in molecules vibrating. These vibrational frequencies depend on the atoms that make up the molecules, how they are connected with their neighbors, and even the shape the molecule is bent into. Direct absorption spectroscopy involves shining the light on the sample and looking for missing photons. The colors that molecules absorb tell us the vibrational frequencies of the molecule.

This works really well with a population of identical molecules. But when you get down to just a few molecules, you are trying to detect a few missing photons in the background of a very bright light. This is simply not possible.

Raman spectroscopy takes an alternative approach. We put in comparatively high energy photons, which scatter off the molecule. Every now and again, one is absorbed and excites vibrational motion. The leftover energy is re-emitted as a photon of light with a lower frequency. By looking at the new frequencies of light, we learn the vibrational frequencies of the molecules. The good thing is that you are looking for photons against a dark background. The bad thing is that it takes a lot of photons to generate a very weak Raman spectrum. So, again, it is very difficult to observe Raman scattering from just a few molecules.

Lu and Belkin from University of Austin in Texas have developed a technique that delivers the best of both worlds. It provides efficient excitation of vibrational frequencies while also retaining the best of Raman scattering: detecting the signal against a dark background. How do they do this? Simple, they convert the light absorption into bulk mechanical motion, and then sense that motion through atomic force microscopy.

Atomic force microscopy

Atomic force microscopy is a way of mapping surface features with very high resolution. It consists of a very sharp tip that is scanned over the surface. The forces on the tip and its vibrations are used to map out the valleys and hills on the surface. To do this with excellent spatial resolution requires tips that have very sharp points, so that their response is dominated by an area of just a few square nanometers.

These tips come to points that are much much smaller than the diffraction limit of light: the diffraction limit is the smallest diameter that a light beam can be focused to. This makes AFM much better at seeing small surface features that light. But it is very difficult to identify molecules with AFM, because there is no direct response to the composition of the molecule (just its shape).

In the research described here, the AFM tip is much much finer than the size of the light beam that is shining on the sample, so it can be used to pinpoint exactly where the sample is heating and expanding. Furthermore, the AFM tip can still be used to gather a typical AFM image, so the result is two images: one of the chemistry of the sample and the other of the physical layout of the sample.

When a flash of light is absorbed by a molecule, it is absorbed by exciting a specific vibrational motion. As the atoms in the molecule move, they excite the motion of other atoms. In turn, they excite others, possibly including some in neighboring molecules. In the end, that one bit of energy ends up distributed over a whole bunch of atomic motions over many different molecules. In other words, we heat up the sample, and, as it is heated, it expands.

The expansion doesn't last for long, of course, because the energy continues to spread out, leaving the area cool again. The trick is to detect this expansion as it's happening. Lu and Belkin do this by placing the cantilever of an atomic force microscope (see side bar) over the sample. They then flash the light at the resonant frequency of the cantilever, so the rhythmic expansion and contraction of the sample excites the cantilever motion, which is then detected.

It turns out that this works pretty well, with the researchers demonstrating a spatial resolution of 50nm. That is pretty good, but isn't the few molecule detector that I was looking for. So, what limits this technique? The main limitation results from the diffusion of the heat. Even when you are not over the precise location of the absorption, you will still detect the acoustic waves.

The only way to get around this would be to look at the relative phase of the expansion: it takes time for the heat to travel outwards from the center of the absorption. But with a heat diffusion time of 100ns and a resonant frequency of 10kHz, we are talking about detecting phase differences of one part in 105.

Ultimately, this technique is probably not going to be iteratively improved a few nanometers at a time. Instead, it is likely to take a significant change in how the molecules are excited.

Chris Lee / Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands.